1. Field of the Invention
The present invention relates to a method and system for wireless power supply in which electric power is transmitted and received wirelessly using electromagnetic waves (microwaves).
2. Description of the Related Art
Technologies for supplying electric power wirelessly using electromagnetic waves have been developed rapidly. One such technology is a system that performs magnetic coupling or magnetic resonance at a short distance of several meters or less to generate electromagnetic induction or magnetic resonance, using microwaves in the frequency range of several hundreds of kilohertz to several megahertz.
There is also a system for wireless power transmission using electromagnetic waves in which electric power is transmitted from a power transmission apparatus to a movable power reception apparatus that is located far from the power transmission apparatus. Microwaves in the range of several hundreds of megahertz to several gigahertz can be transmitted to faraway power reception apparatuses. Examples of such systems for wireless power transmission include a space solar power system in which an artificial satellite with solar panels is launched to an altitude above the equator for transmitting solar photovoltaic power to ground-based power reception apparatuses; a system for transmitting power to power reception apparatuses located on an isolated island, and a system for supplying power to mobile objects such as electric vehicles (EVs).
Such systems for wireless power transmission using electromagnetic waves (microwaves) have a higher degree of freedom than other systems in the distance over which a power transmission antenna transmits electric power. For this reason, these systems have been proposed to supply and charge electric power to mobile objects such as electric vehicles and robots.
These systems for power transmission, however, undesirably degrade their power supply efficiency due to misalignment in the position of power supply. Another problem with these systems is the lack of safety of electromagnetic waves leaking outside and their influence on other devices (EMI: Electro-Magnetic Interference) due to the use of high microwave frequencies. To achieve accurate positioning by overcoming the former problem, the following approaches have been proposed. Japanese Unexamined Patent Publication No. 2013-236524 discloses that the position of a mobile object on the power reception side is recognized and the mobile object is guided to the position of power supply by the power transmission side. Meanwhile, Japanese Unexamined Patent Publication No. 2008-92703 discloses that a device on the power transmission side is mechanically controlled to be aligned with a power reception antenna and to achieve power transmission.
FIG. 9 shows a configuration example of a conventional system for power transmission. Microwave generator 101 supplies microwave power to power transmission antenna 103 through microwave transmission line 102. Microwave transmission line 102 includes a distributor, and in some cases, an amplifier. Power transmission antenna 103 has a power transmission surface large enough to cover the movable range (the misalignment range) of power reception antenna 105. The power transmission surface of power transmission antenna 103 has opening 104 corresponding only to the power reception surface of power reception antenna 105. An electromagnetic wave is radiated only through opening 104 to achieve efficient microwave power transmission. Power reception antenna 105 receives the electromagnetic wave, which is converted into appropriate power by power conversion circuit 106 and then supplied to power supply object 107.
Examples of proposed methods for recognizing the switching position of the opening of a power transmission antenna are as follows. In one example, the position of the opening of the power transmission antenna is controlled by recognizing the position of the power reception antenna by a mechanical operation of a magnet. In another example, antenna elements used for power transmission are electrically switched by recognizing the position of the power reception antenna by using a separate component.